1. Field of the Invention:
The present invention relates an apparatus for controlling an elevator that is driven by an induction motor and, more particularly, to a process of regenerative power.
2. DESCRIPTION OF THE PRIOR ART
In an A-C elevator in which a commercial A-C power source is converted by a converter into a D-C power, the D-C power is then converted by an inverter into an A-C power of variable voltage and variable frequency to drive an induction motor for hoisting the cage of the elevator, the regenerative power generated from the elevator is consumed in the motor. An example of such an A-C elevator has hitherto been proposed in Japanese Patent Application Laid-open No. 59-17879.
FIGS. 12 and 13 show the prior-art apparatus for controlling the A-C elevator mentioned above. FIG. 12 is a view of the construction, and FIG. 13 is a simple equivalent circuit of the induction motor.
In FIG. 12, numeral 1 indicates three-phase A-C power sources, numeral 2 designates circuit breakers connected to the A-C power sources 1, numeral 3 denotes a converter, which comprises three-phase full-wave rectifiers having diodes connected to the circuit breakers 2, numeral 4 indicates a smoothing capacitor connected to the D-C side of the converter 3, numeral 5 designates an inverter connected to the smoothing capacitor 4 and having a transistor and a diode for converting the D-C power into three-phase A-C powers of variable voltage and variable frequency, numeral 6 denotes a three-phase induction motor connected to the A-C side of the inverter 5, numeral 7 indicates a drive sheave of a hoist driven by the motor 6, numeral 8 designates a main cable engaged with the sheave 7 and coupled at respective ends thereof with a cage 9 and a balance weight 10, numeral 11 denotes a pulse generator coupled directly with the motor 6 for detecting the rotating speed of the motor 6, numeral 12 indicates an inverter drive unit controlled by control means 13 for driving the inverter 5.
The prior-art apparatus for controlling the A-C elevator is constructed as described above, and when the circuit breaker 2 is closed, the A-C power source 1 is connected to the converter 3, which rectifies the three-phase power into D-C power, and the D-C power is smoothed by the smoothing capacitor 4. When a start command is applied to the cage 9, the inverter drive unit 12 operates, the inverter 5 converts the D-C power into three-phase A-C power of variable voltage and variable frequency, and supplies the A-C power to the motor 6. Thus, the motor 6 rotates to elevate the cage 9. On the other hand, the rotating speed of the motor 6 is detected by the pulse generator 11, a speed signal at the output of the generator 11 is fed back to control the inverter drive unit 12, thereby accurately controlling the cage 9 to run at the desired speed.
The detailed control of the prior-art apparatus is omitted, but, in summary, when the elevator is operated in the power mode, the torque of the motor 6 is controlled by so-called "slip frequency control", but when the elevator is regeneratively operated, the regenerative power is consumed in the motor 6 as will be described herebelow.
The power P.sub.1 consumed in the motor 6 will be obtained from an equivalent circuit shown in FIG. 13. ##EQU1## wherein: V denotes an A-C input power voltage,
Z denotes total impedance of the motor, PA1 g.sub.0 denotes exciting conductance of the motor, PA1 b.sub.0 denotes exciting susceptance of the motor, PA1 r.sub.1 denotes primary resistance of the motor, PA1 r.sub.2 denotes the secondary resistance of the motor (primary conversion value), PA1 x.sub.1 denotes the primary leakage reactance of the motor, PA1 x.sub.2 denotes the secondary leakage reactance of the motor (primary conversion value), PA1 S denotes the slip of the motor.
On the other hand, the power Pg generated as the regenerative power is given by, ##EQU2## wherein: is the slip S is controlled to become: EQU P.sub.1 +Pg=0 (4)
the regenerative power is completely consumed in the motor.
The equations (1) and (3) are substituted in the equation (4) to obtain the slip S. ##EQU3##
Normally, since the exciting conductance g.sub.0 is sufficiently small, assuming that the exciting conductance g.sub.0 is ignored, the equation (5) can be simplified as below. ##EQU4##
Therefore, when the slip is controlled according to the equation (6), the regenerative power can be completely consumed in the motor. In this case, the regenerative power Pg.sub.1 is given as below by substituting the equation (6) in the equation (3), ##EQU5## The physical meaning of the equation (7) exhibits that the regenerative power is all consumed in the primary resistance r.sub.1 and the secondary resistance r.sub.2.
Assume that the slip S is given by, ##EQU6## and if (R.sub.1 &gt;1) is obtained, the regenerative power Pg.sub.2 in this case is give by, ##EQU7## and, since R.sub.1 &gt;1 exists, the regenerative power more than consumed in the motor 6 is returned.
On the contrary, if the slip S is given by, ##EQU8## the regenerative power Pg.sub.3 in this case is given by, ##EQU9##
and, since R.sub.2 &gt;1 exists, the power larger than the regenerative power is consumed in the motor 6.
More specifically, the power is further supplied from the power source 1.
In summary, if the slip S is given by, ##EQU10## a problem arises in that the regenerative power is returned to the power source side. If the slip S is given by, ##EQU11## since the power is further supplied from the power source, the heat generated from the motor 6 increases. Then, ##EQU12## is the condition for balancing between the regenerative power and the consumed power in the motor 6.
In the abovementioned prior-art apparatus for controlling the A-C elevator, it is not easy to operate the elevator with the slip S to be fixed to the equation given by, ##EQU13## This is because it is difficult to accurately measure the resistance of the primary resistance r.sub.1 and the secondary resistance r.sub.2. Further, the resistance values largely alter according to the temperature. Thus, the slip S can hardly be controlled according to the condition given by, ##EQU14## and, if the condition is displaced, the regenerative power is returned to the power source side, or the heat generated in the motor 6 increases as drawbacks.